1. Field of the Invention
The present invention relates to an optical semiconductor device provided with a load resistance for matching, and a method of fabricating the same.
2. Description of the Related Art
Active research and development activities have been made to use high modulation frequencies of 10 GHz or above for the rapid transmission of large quantity of information. Optical communication uses photodiodes, i.e., photoelectric converters. A photodiode receives light signals traveled through an optical fiber and converts the same into corresponding electric signals. An electronic circuit processes the electric signals and converts the same into characters, images or sounds.
The impedance of the electronic circuit with respect to the photodiode, and the impedance of the photodiode with respect to the electronic circuit must be accurately matched. A resistor is used for matching the impedances. Conventional impedance matching uses a resistor of an alloy having a comparatively high resistivity, such as NiCr, or an n-type semiconductor having a high impurity concentration.
Impedance matching must be performed in an accuracy higher than that in which conventional impedance matching is performed, when light signals of high frequencies higher than a microwave band for frequencies on the order of several gigahertz. To achieve impedance matching in such a high accuracy, the resistor must be formed so that its resistance coincides accurately with a design resistance, and hence the resistor must be formed in a comparatively big width.
The resistivities of NiCr and n-type semiconductors conventionally used for forming resistors are not very high. Since the resistance of a resistor is proportional to the resistivity of the material forming the resistor and inversely proportional to the width of the resistor, the resistor must be formed in a small width of several micrometers. However, the resistance of a resistor having a small width changes greatly when the width changes slightly, which makes matching difficult. Reduction of the width of a resistor increases the inductance of the resistor, which affects adversely to matching.
Accordingly, it is an object of the present invention to provide an optical semiconductor device capable of solving the foregoing problems, and a method of easily fabricating such an optical semiconductor device.